The present invention relates to a novel line encoding scheme and communication system for transmission of digital data over a transmission link.
Various line encoding techniques have been proposed in the art of digital communications to minimize the effect of transmission impairments on the transmitted signal. These impairments are typically in the form of interference occurring between symbols, known as intersymbol interference, or in the transmission-induced jitter of a clock signal. Conventional encoding schemes and communication systems convert digitally encoded data, for instance NRZ-type data, into a sequence which requires transmission of a dedicated clock signal. The clock signal is transmitted simultaneously with the data, usually on two separate lines, or it may be embedded in the data signal.
In order to receive or decode such transmissions, complex, expensive electronic schemes and equipment are necessary. For example, where the clock signal is embedded in the data signal, the transmitted data stream undergoes a full-wave rectification or other non-linear operation and is recovered through bandpass filters having a frequency centered around the bit rate. Alternatively, many conventional schemes pass the data stream through phase-locked loops having a fixed level crossing reference, or other similar analog frequency detecting circuits.
These conventional schemes suffer the disadvantages that not only are they complex and costly to implement, but depend directly on the data transmission rate, which must be known either prior to transmission or prior to system design. These restrictions render conventional fixed-rate synchronization line encoding techniques useless in systems where the user's data rate is unknown in advance, or if services with different data rates need to share the same channel.
Moreover, the spectral characteristics of these techniques are not ideally suited for transmission over fiber-optic or other communication links and suffer from transmission-induced signal degradation while requiring precise frequency and clock recovery circuits.
There exists, therefore, a need for a line coding scheme and communication system which is absolutely transparent to the rate of the data being transmitted, both as to transmission and receiving of the data and which provides more favorable spectral characteristics for transmission of digital data over a fiber-optic link.